1. Field of the Invention
The present invention generally relates to a secondary air supplier for use in a vehicle. More particularly, the present invention pertains to a diagnostic system for diagnosing the condition of the secondary air supplier. The air supplier supplies secondary air to an exhaust passage of the engine, in order to purify the exhaust gas.
2. Description of the Related Art
Conventional exhaust gas purification techniques generally utilize an oxygen sensor and a three way catalytic converter, in order to satisfy two requirements. The first requirement is to clear the legalistic regulation, and the other requirement is to minimize the engine fuel consumption. These conventional techniques include oxidizing and/or deoxidizing the carbon monoxide (CO), hydro-carbon (HC) and nitrogen oxides (NOx), contained in the exhaust gas, in order to purify it. In order to efficiently complete these reactions, the air-fuel ratio in the engine should maintained at about a theoretical air-fuel ratio. The concept for maintaining the theoretical air-fuel ratio has been embodied in several engine systems.
An exemplary engine system of this type includes an engine control unit, an oxygen sensor disposed along an exhaust passage, and a secondary air supplier for directly supplying the secondary air from an intake passage to the exhaust passage. The control unit calculates the air-fuel ratio (A/F) of the air-fuel mixture in the engine cylinders, based on the output signal from the oxygen sensor. Further, the control unit executes a closed loop control operation (i.e., feedback control operation) for controlling the supplied amount of air and/or fuel. As a result of this control, the air-fuel ratio becomes closer to the theoretical air-fuel ratio.
When the engine is operating under specific conditions, the secondary air is supplied to the exhaust passage by means of the secondary air supplier, in order to prevent air-fuel ratio to be fuel-rich condition, and to minimize the generation of hydrocarbon.
The following are illustrative specific operation conditions of the engine: the coolant temperature is still cold; and the vehicle is decelerating. The control unit executes an open loop control operation for the air-fuel ratio (A/F), while the secondary air is being supplied. The control unit re-executes the closed loop control operation for the air-fuel ratio, simultaneously with the termination of the secondary air supply.
In this conventional engine system, the exhaust emission could become faulty, when the secondary air supplier malfunctions. TOYOTA technology publication No. 2801 (published on Oct. 28, 1988) proposed a diagnostic system for diagnosing whether or not the secondary air supplier is functioning properly.
A conventional secondary air supplier temporarily supplies the secondary air to the exhaust passage, even when the secondary air should not be supplied to the exhaust passage. If the secondary air supplier is properly functioning, the oxygen sensor should transmit a signal which indicates the air-fuel ratio (A/F) is "lean", that is the air content is greater then the fuel content. If the oxygen sensor does not transmit a signal indicative of the lean condition of the air-fuel ratio, the secondary air supplier might not function properly. Therefore, in this case, the conventional diagnostic system determines that the secondary air supplier is malfunctioning.
The Japanese Unexamined Patent Publication No. 63-111256 discloses another system for diagnosing the secondary air supplier. When the oxygen sensor is transmitting a signal, which indicates the air-fuel ratio (A/F) is rich condition, that is the fuel content is greater than the air content, under specific engine conditions under which the secondary air should be supplied, the diagnostic system determines that the secondary air supplier is malfunctioning.
However, both conventional diagnostic systems might sometime determine that the secondary air supplier is malfunctioning, even when the oxygen sensor transmits a signal indicative of the rich condition of the air-fuel ratio (A/F), in response to the temporarily rich condition of the air-fuel ratio. Some examples of misdiagnosis will now be described.
The output signal transmitted from the oxygen sensor has characteristic of: radical change of signal state being made around the specific point of A/F (i.e., 14.6). Therefore, even when the air-fuel ratio temporarily becomes "14.5" (this value indicates that the air-fuel ratio is slightly rich), the diagnostic system thus determines that the air-fuel ratio is positively rich (i.e., the supplier is malfunctioning).
The exhaust gas discharged from the engine cylinders is mixed within the exhaust passage. Only one oxygen sensor determines the air-fuel ratio (A/F) in the exhaust gas. Therefore, when the air-fuel ratio of the exhaust gas discharged from the cylinders are differed, the oxygen sensor could give a fault indication, based on the exhaust gas discharged from a particular.
In other words, even when the mean value of the air-fuel ratio in the exhaust gas discharged from all cylinders, is lean, if the oxygen sensor detects a rich air-fuel ratio is in the rich condition, the diagnostic system then determines that the supplier is malfunctioning.
If the air-fuel ratio of the mixture in the combustion chambers is extremely rich, the air-fuel ratio of the exhaust gas could remain rich, even if secondary air is supplied to the exhaust gas. This condition could occur in any one of following conditions: (1) an additional amount of injected fuel is excessively large when the engine is still cold; (2) immediately after the simultaneous injection of fuel is executed; or (3) immediately after the vehicle starts decelerating.
Further, when the amount of exhaust gas is too large with respect to that of the secondary air, the air-fuel ratio (A/F) of the exhaust gas could become rich. Even under those conditions, the diagnostic system executes the diagnostic operation, only based on a signal transmitted from the oxygen sensor. Therefore, even when the secondary air supplier is functioning properly, the diagnostic system determines that the supplier is malfunctioning.
Furthermore, the oxygen sensor tends to transmit a signal indicative of the lean condition, when its sensing element is cold. As a result, even if the air-fuel ratio (A/F) is actually rich, the diagnostic system could determine that the air-fuel ratio (A/F) is lean, due to the cold element temperature of the sensing element. In such a case, the secondary air supplier should be diagnosed as malfunctioning, but the diagnostic system determines that the supplier is functioning properly.
The conventional diagnostic system determines whether or not the supplier is functioning properly, only based on the signal output from the oxygen sensor. Therefore, the conventional diagnostic system could misdiagnose the condition of the secondary air supplier, as described above.